The present invention relates generally to drop-on-demand liquid emission devices, and, more particularly, to ink jet devices which employ thermo-mechanical actuators.
Drop-on-demand (DOD) liquid emission devices have been known as ink printing devices in ink jet printing systems for many years. Early devices were based on piezoelectric actuators such as are disclosed by Kyser et al., in U.S. Pat. No. 3,946,398 and Stemme in U.S. Pat. No. 3,747,120. A currently popular form of ink jet printing, thermal ink jet (or xe2x80x9cbubble jetxe2x80x9d), uses electroresistive heaters to generate vapor bubbles which cause drop emission, as is discussed by Hara et al., in U.S. Pat. No. 4,296,421.
Electroresistive heater actuators have manufacturing cost advantages over piezoelectric actuators because they can be fabricated using well developed microelectronic processes. On the other hand, the thermal ink jet drop ejection mechanism requires the ink to have a vaporizable component, and locally raises ink temperatures well above the boiling point of this component. This temperature exposure places severe limits on the formulation of inks and other liquids that may be reliably emitted by thermal ink jet devices. Piezo-electrically actuated devices do not impose such severe limitations on the liquids that can be jetted because the liquid is mechanically pressurized.
The availability, cost, and technical performance improvements that have been realized by ink jet device suppliers have also engendered interest in the devices for other applications requiring micro-metering of liquids. These new applications include dispensing specialized chemicals for micro analytic chemistry as disclosed by Pease et al., in U.S. Pat. No. 5,599,695; dispensing coating materials for electronic device manufacturing as disclosed by Naka et al., in U.S. Pat. No. 5,902,648; and for dispensing microdrops for medical inhalation therapy as disclosed by Psaros et al., in U.S. Pat. No. 5,771,882. Devices and methods capable of emitting, on demand, micron-sized drops of a broad range of liquids are needed for highest quality image printing, but also for emerging applications where liquid dispensing requires mono-dispersion of ultra small drops, accurate placement and timing, and minute increments.
A low cost approach to micro drop emission is needed which can be used with a broad range of liquid formulations. Apparatus and methods are needed which combines the advantages of microelectronic fabrication used for thermal ink jet with the liquid composition latitude available to piezo-electro-mechanical devices.
A DOD ink jet device which uses a thermo-mechanical actuator was disclosed by T. Kitahara in JP 2030543, published Jan. 31, 1990. The actuator is configured as a bi-layer cantilever moveable within an ink jet chamber. The beam is heated by a resistor causing it to bend due to a mismatch in thermal expansion of the layers. The free end of the beam moves to pressurize the ink at the nozzle causing drop emission. Recently, disclosures of a similar thermo-mechanical DOD ink jet configuration have been made by K. Silverbrook in U.S. Pat. Nos. 6,067,797; 6,234,609; and 6,239,821. Methods of manufacturing thermo-mechanical ink jet devices using microelectronic processes have been disclosed by K. Silverbrook in U.S. Pat. Nos. 6,254,793 and 6,274,056.
Thermo-mechanically actuated drop emitters are promising as low cost devices which can be mass produced using microelectronic materials and equipment and which allow operation with liquids that would be unreliable in a thermal ink jet device. However, operation of thermal actuator style drop emitters, at high drop repetition frequencies, requires careful attention to excess heat build-up. The drop generation event relies on creating a pressure impulse in the liquid at the nozzle. A significant rise in baseline temperature of the emitter device, and, especially, of the thermo-mechanical actuator itself, precludes system control of a portion of the available actuator displacement that can be achieved without exceeding maximum operating temperature limits of device materials and the working liquid itself. Apparatus and methods of operation for thermo-mechanical DOD emitters are needed which manage heat build-up so as to maximize the productivity of such devices.
The present invention provides for emitting drops by reducing the thermal energy input when groups of drops or certain series of drops are required by the application. A damped resonant oscillation of the thermal actuator itself is required to implement the present invention.
Use of fluid resonances is known for piezoelectric drop-on-demand ink jet devices. In these known methods, the resonance of the ink meniscus at the nozzle, driven by surface tension effects, or the Helmholtz resonance of the ink chamber, driven by compliance effects, is used to change the volume or number of emitted drops. Tence et al. in U.S. Pat. No. 5,689,291 employ waveforms that drive piezoelectric transducers with spectral energy concentrations at frequencies associated with modal resonances of ink in the ink jet printhead orifices. Exciting different resonance modes of the ink meniscus causes the emission of different drop sizes.
Paton et al., in U.S. Pat. No. 5,361,084, disclose a method of multi-tone printing using a piezoelectric DOD printhead having elongated ink chambers and sidewall actuators, wherein an individual jet is excited using a packet of pulses so as to excite a longitudinal acoustic resonance in the jet channel which causes the emission of a number of discrete drops. Lee et al., in U.S. Pat. No. 4,513,299 disclose a similar use of acoustic resonance of the ink channels of a piezoelectric ink jet printhead.
DeBonte et al., in U.S. Pat. No. 5,202,659, disclose a method of operating a piezoelectric printhead using the dominant resonant frequency of the ink jet apparatus. In the specific examples disclosed, this dominant resonance is described as the Helmholtz resonance of an individual jet chamber which is excited by actuating the piezo transducer to first expand the jet chamber, waiting the resonance period, and then contracting the chamber to reinforce this resonance. This excitation process is repeated for multiple cycles to generate multiple merging drops for printing spots having different sizes. The methods disclosed by DeBonte, et al., operate by exciting bulk fluid mechanical resonances which are appropriate for physically large piezoelectric devices but are not useful for drop emitter chambers fabricated with dimensions of less than a few hundred microns. Helmholtz and other fluid mechanical resonances occur at frequencies too high to support multiple drop formation when fluid path dimensions are less than 1 mm.
Other piezoelectric ink jet inventors discourage using fluid mechanical resonances when uniformity of drop volume and velocity are important. Stanley et al., in U.S. Pat. No. 5,170,177 discloses a method of operating a piezo DOD device wherein the electrical pulses are adjusted to minimize their energy content at a frequency corresponding to the dominant acoustic frequency of the ink jet in order to accelerate drop break-off, optimize drop shape and minimize drop speed variations. Disclosures by Murakami et al., in U.S. Pat. No. 4,577,201 and Torii et al., in U.S. Pat. No. 6,102,512 also teach avoidance of Helmholtz and other fluid mechanical resonances in order to achieve uniform drop volume and velocity performance. Further, Pengelly in U.S. Pat. No. 5,801,732 discloses drop volume and velocity non-uniformities caused by exciting piezo transducer resonances in an ink jet printhead and a timing method for reducing these effects.
Thermo-mechanical DOD emitters are needed which reduce energy input and waste heat build-up so as to allow maximum net drop emission frequencies. The present invention makes use of a damped resonance oscillation of the thermal actuator and not fluid mechanical resonances of the meniscus, Helmholtz oscillations of fluid chambers, or other resonances of the overall ink jet apparatus. The small dimensional scale of microfabricated cantilever actuators provides resonant frequencies in a range useful for the fine and ultra fine drop emitters operated by the apparatus and methods of the present invention.
It is therefore an object of the present invention to provide a liquid drop emitter which is actuated by a thermo-mechanical cantilever.
It is also an object of the present invention to provide a thermo-mechanical drop emitter to produce series and groups of droplets having substantially equal volume and velocity.
It is further an object of the present invention to provide a thermo-mechanical drop emitter using reduced input energy for some drops thereby reducing overall energy usage, managing heat build-up and allowing increased productivity of drop emission.
Yet another object of the present invention is to operate a thermo-mechanical cantilever in a damped resonant mode making advantageous use of the oscillating motion of the thermal actuator subsequent to a drop emission in order to emit a next drop using less electrical energy.
The foregoing and numerous other features, objects and advantages of the present invention will become readily apparent upon a review of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by operating a liquid drop emitter for emitting a series of liquid drops having substantially uniform volume and velocity, wherein the drop emitter comprises a liquid-filled chamber having a nozzle and a thermal actuator for applying pressure to liquid at the nozzle. The thermal actuator is configured as a cantilever with a free end that is movable within the chamber. The thermal actuator exhibits a damped resonant oscillation having a fundamental period, TR, and a damping time constant, TD. The thermal actuator further comprises electroresistive heater means that suddenly heat the thermal actuator in response to electrical pulses. The sudden heating causes bending of the thermal actuator, the displacement of the free end, and pressurization of the liquid at the nozzle sufficient to cause drop ejection. A source of electrical pulses is connected to the liquid drop emitter and a controller means receives commands to emit drops and determines the timing and parameters of the electrical pulses which are applied to the liquid drop emitter. The method of operating comprises the steps of (a) applying to the electroresistive heating means a nominal electrical pulse having a nominal energy, E0, and a nominal pulse duration, TP0, where TP0 less than xc2xd TR so that a nominal drop is emitted and a damped resonant oscillation of the thermal actuator is initiated. After (b) receiving a command to emit a next drop, it is determined if the thermal actuator is usefully oscillating. If not, the method returns to (a). If so, a waiting time, Tw, is observed until the thermal actuator is moving so as to pressurize the liquid at the nozzle. And, then, a next electrical pulse having a second energy, E2, where E2 less than E0, and a second pulse duration, TP2, where TP2 less than xc2xdTR, is applied to the electroresistive heating means so that a next drop having substantially the same volume and velocity as the nominal drop, is emitted. The application of reduced energy pulses is timed to reinforce the first, second, or third resonant oscillation of the thermal actuator. The amount of energy reduction accomplished depends, at least, on which resonant oscillation is useable and on the damping time constant, TD.
The present invention is particularly useful for operating liquid drop emitters for DOD ink jet printing. In this embodiment, image data is organized as a series of pixels to be printed by a given jet, including many clusters of adjacent pixels. Therefore, many drop emissions will be eligible for reduced energy pulses.